In Long Term Evolution (LTE) and LTE-advanced standardizations in Third Generation Partnership Project (3GPP), discontinuous reception (DRX) control is performed for the purpose of battery energy saving of user equipment (UE). See, for example, Non-Patent Document 1 listed below.
In DRX control, mechanisms for allowing a UE to transit to inactive states as smooth as possible are introduced. One of them is a hybrid automatic repeat request round-trip time (referred to as a “HARQ RTT”) timer. The HARQ RTT timer is used to determine timing for data retransmission during reception of downlink data. As long as the HARQ RTT timer is running, UE does not have to become the active mode for at least the data retransmission because it is unlikely to receive retransmitted data until the timer is up. Although a UE may be in the active state during the duration of the HARQ RTT timer counting due to other conditions, such exceptional conditions are excluded in this context.
Meanwhile, in LTE Release 10 and the subsequent releases, carrier aggregation (CA) for aggregating multiple component carriers (CCs) supported by the same base station or evolved node B (eNB) has been specified to improve the throughput. This type of carrier aggregation may be named “intra-eNB CA”. In Releases 10 and 11, only those component carriers operated in the same duplex mode are aggregate by carrier aggregation. In other words, the component carriers supported at the primary cell (PCell) and the secondary cell (SCell) respectively are operable in the same duplex mode, namely, both in frequency division duplexing (FDD) mode or both in time division duplexing (TDD) mode.
In Release 12, TDD-FDD carrier aggregation is discussed, which technique expands the conventional CA to carrier aggregation between different duplex modes. See, for example, Non-Patent Document 2 listed below.